For about 25 years it was thought that this cartoon had been lost forever. Then out of nowhere it was uploaded to Youtube 2 months ago. The internet is really great for the survival of rare art. Even if only one copy of a work exists, once it's online you've essentially got an unlimited number of potential copies.
I can see why Mickey would've thought he was safe. Cartoon physics usually dictates that gunshots merely blacken the target (usually their head) for a few moments, allowing the individual to return to normal after two eye-blinks that sound like someone plinking some high-register piano keys.
As these results were often coupled with racist "blackface" sight gags, perhaps the 60's had altered the laws of cartoon behavior, which allowed this unfortunate individual to expire.
Cartoon physics ceased to be applied when the mice segregated themselves to Mouseville during the White Flight From Duckburg of the Eisenhower administration. The ducks are still subject to cartoon physics to this very day, but mice are historically allowed the privilege of death, the only hope that exists in the candy colored world of animation.